Combustors have long been used to burn a fuel/air mixture that is ultimately used to generate thrust, produce power, supply heat for some industrial process, or other applications. In these systems, an important performance metric is for the flame to remain stably in the combustor over a range of flow rates, pressures, and fuel/air ratios. At certain conditions, however, the flame may “blow out” of the combustor, so that no flame exists. The problem of blowout has long limited the allowable flow velocities through engines, particularly in systems such as gas turbines and afterburners which must operate at high flow rates and/or low pressures. The problem of blowout, however, has become increasingly more severe in a range of combustion devices, as they are required to meet stringent emissions legislation, severe operability constraints, and achieve better performance.
The problem of flame blowout can occur in combustors of land-based turbine engines, aeronautical turbine engines, afterburners, industrial processing devices, or any other combustor device. With respect to land-based turbine engines, operators of such engines attempt to run the engine near flame blowout conditions, known as the lean blowout line. An advantage of operating so close to the blowout line is that nitrous oxide emissions are significantly lowered. The trade-off, however, is an increased likelihood of blowing out of the flame. In the land-based systems, a blow out event requires a potentially lengthy system shut down and restart, resulting in economic consequences to the power plant owner when blowout is encountered.
In the aeronautical setting, blowout is a particular concern during fast engine transients, such as when rapid acceleration or deceleration of the engine is attempted. If the flame blows out in a commercial airplane, then there are obvious safety concerns for the passengers, though most engines can be re-ignited in-flight. However, because of the magnitude of the possible consequences, engine designers include substantial safety margins into the engines to avoid these events, often at the cost of reduced performance in other areas.
The need to avoid blowout in combustors often causes designers to sacrifice performance in other areas. In particular, because there is always some uncertainty in the exact conditions under which blowout may occur, extra margin must be built into the design. In such systems, performance could be improved and blowout better avoided if a method existed to monitor the proximity of the system to blowout.
A method designed to predict blowout conditions is U.S. Pat. No. 5,706,643 to Snyder et al. The Snyder patent discloses a method for predicting blowout conditions to minimize nitrous oxide emissions in land-based turbine engines. Snyder uses pressure measurements in the combustor to predict the onset of blowout conditions by analyzing pressure oscillations. The methods consist of monitoring the magnitude of the pressure, certain spectral components of the pressure, or the dominant frequency of the pressure. However, the methods rely on monitoring absolute magnitudes of the pressure signal, which may change on other engines, at different power settings, or due to inherent variability in pressure, temperature, or humidity of the air. As such, the methods reported by Snyder are designed to operate upon a particular engine at a particular operating condition. In addition, the dominant frequency may also change with engine type or operating conditions. Thus, the methods employed by Snyder are not robust and seemingly are operable only on the particular type of combustor tested and only under certain operating conditions. The methods taught by Snyder are not expansive to different combustor types operating in a wide array of environmental conditions.
Thus, there exists a need in the industry for a system and method for accurately predicting flame blowout conditions on different types of combustors operating in different environments.
Once flame blowout conditions are accurately predicted the combustor needs to be controlled to prevent flame blowout. Current systems in the industry have preset operating inputs, such as fuel flow and air flow, for specific load conditions. But no system or method exists in the industry that implements a closed-loop control system to actively change parameters in the combustor on a real time basis to prevent flame blowout based upon knowledge of the flame's stability characteristics.